Health BMR calculator
Calculate basal metabolic rate using Mifflin-St Jeor, Harris-Benedict or Katch-McArdle, and see total daily burn at every activity level.
Basal metabolic rate
1,649 kcal
Energy burned at complete rest, before any movement at all.
- Mifflin-St Jeor
- 1,649 kcal
- Harris-Benedict
- 1,696 kcal
- Katch-McArdle
- 1,580 kcal
Total daily burn by activity level
BMR is typically 60-70% of what you burn in a day. The multiplier is the weakest link here, since most people place themselves one category too high.
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Reembun. (2026, July 28). BMR calculator. https://reembun.com/bmr-calculator
How to use it
Fill in the basics
Unit system first, then sex at birth, age, height and weight.
Pick an equation
Mifflin St Jeor is the default and the closest for most people. Katch McArdle asks for your body fat percentage and repays you for knowing it.
Read the basal rate
The figure is what your body burns over 24 hours at complete rest, before any movement is counted.
Turn it into a daily target
Multiply it by an activity factor, or use the calorie calculator, which does that step for you.
The three equations
Mifflin-St Jeor (1990) is the current default, and the one the Academy of Nutrition and Dietetics found most reliable for both normal-weight and obese adults.
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161Harris-Benedict, revised (1984) takes the original 1919 equations as revised by Roza and Shizgal. Tends to run about 5% high.
Men: BMR = 88.362 + (13.397 × kg) + (4.799 × cm) − (5.677 × age)
Women: BMR = 447.593 + (9.247 × kg) + (3.098 × cm) − (4.330 × age)Katch-McArdle works from lean body mass, so it needs a body fat percentage. When you have a reliable one, this is the most accurate of the three, because it sidesteps the problem that total weight cannot distinguish muscle from fat.
BMR = 370 + (21.6 × lean mass in kg)A worked example
A 30-year-old man, 175 cm, 70 kg, 15% body fat:
| Equation | BMR |
|---|---|
| Mifflin-St Jeor | 1,649 kcal |
| Harris-Benedict | 1,724 kcal |
| Katch-McArdle | 1,655 kcal |
The 75 kcal spread between the first two is about 4.5%, consistent with Harris-Benedict’s known bias, and small enough that it matters less than the activity multiplier applied next.
From BMR to total daily burn
BMR is typically 60-70% of what you burn in a day. The rest comes from movement, exercise and the energy cost of digestion.
| Activity level | Multiplier | On 1,649 BMR |
|---|---|---|
| Sedentary: desk job, little exercise | 1.2 | 1,979 kcal |
| Light: exercise 1-3 days a week | 1.375 | 2,267 kcal |
| Moderate: exercise 3-5 days a week | 1.55 | 2,556 kcal |
| Active: exercise 6-7 days a week | 1.725 | 2,845 kcal |
| Very active: physical job or twice-daily training | 1.9 | 3,133 kcal |
The multiplier is the weak link
The BMR equations are fitted to measured data. The multipliers are broad categories, and the evidence is consistent that people place themselves too high, often by a whole category.
If your weight is not moving as predicted after two to three weeks of consistent intake, the multiplier is the thing to revise, not the equation. Track weight as a weekly average rather than a daily reading, since day-to-day swings are mostly water and gut contents.
What the equations cannot see
Body composition. Two people identical in height, weight, age and sex can differ by 200-300 kcal a day depending on muscle mass. This is exactly what Katch-McArdle addresses.
Thyroid function and medication. Hypothyroidism, hyperthyroidism and a range of common medications shift metabolic rate materially. No predictive equation accounts for them.
Adaptation. Sustained restriction reduces both BMR and non-exercise activity. Recalculating every 4-5 kg of change keeps the estimate honest.
Genetics. Measured variation between individuals of identical body composition is real, and no equation captures it.
Very low intakes are counterproductive and can be dangerous. Sustained intake below roughly 1,500 kcal for men or 1,200 kcal for women makes adequate protein and micronutrients hard to reach and should only happen under clinical supervision.
Common questions
Which equation should I use?
Mifflin-St Jeor unless you know your body fat percentage, in which case Katch-McArdle is more accurate because it works from lean mass rather than total weight. Harris-Benedict is included for comparison but is known to overestimate by roughly 5%. It was derived in 1919 from a population that differed from today's.
What is the difference between BMR and RMR?
BMR is measured after an overnight fast, at complete rest, in a thermally neutral room, awake but motionless. RMR is measured under less strict conditions and comes out about 10% higher. Predictive equations are technically estimating BMR but are commonly used interchangeably with RMR, and the difference is smaller than the equations' own error margin.
How accurate are these equations?
Mifflin-St Jeor predicts measured resting energy expenditure within 10% for roughly 80% of people. That means one person in five is more than 10% out, in either direction. Use the number as a starting point and adjust based on what actually happens to your weight over two to three weeks.
Why does BMR fall when I lose weight?
Two reasons. A smaller body needs less energy to maintain, which is arithmetic. But sustained deficits also cause adaptive thermogenesis: the body reduces energy expenditure beyond what the weight loss alone predicts, partly through reduced spontaneous movement. This is why weight loss slows even when adherence does not.
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